Variable pulley transmissions for transferring torque from an input or drive shaft to an output or driven shaft have been used for some time. In these transmissions, a first pulley constructed of a pair of flanges is mounted on the input shaft such that at least one of the flanges is axially movable with respect to the other flange. A second, similar pair of pulley flanges are mounted on the output shaft. A flexible belt connects the two pulleys to transfer torque from the input shaft to the output shaft. As the effective diameters of the pulleys are simultaneously and inversely varied, the drive ratio between the input and output shafts is adjusted in a smooth, continuous manner.
Automotive engineers recognize that the operating efficiency of an internal combustion engine be enhanced by adjusting the transmission to varying loads and speed ratios. This enhancement has not been possible in a conventional, fixed-ratio gear transmission and engine arrangement. In a conventionally geared transmission, the drive ratio is adjusted in discrete steps, rather than continuously over the operating range. Accordingly, recent research efforts have been directed to the use of a continuously variable transmission (CVT). These efforts have resulted in the development of passenger cars using a flexible and continuous rubber belt to drivingly interconnect the pulleys. Rubber belts are subject to wear from the torque load, temperature, vibration and other adverse conditions. Development efforts are continuing to improve the belt life and to produce a flexible belt of metal, which efforts are described in the literature.
Flexible metal belts for use with CVTs are frequently categorized as "push" belts and as "pull" belts. An example of a push belt is described in U.S. Pat. No. 3,720,113--Van Doorne et al. Similarly, a pull belt is described in U.S. Pat. No. 4,313,730--Cole, Jr. et al. The Van Doorne et al--'113 belt comprises an endless carrier with a plurality of nested metal bands and an endless array of generally trapezoidal (when viewed from the front) load blocks encircling the carrier and longitudinally movable therewith. Each block has edge surfaces to frictionally engage the pulley flanges of a pulley transmission to transmit torque between the pulleys.
The pull belt of Cole, Jr. et al--'730 utilizes an endless chain as the carrier, the sets of links of which are pivotably interconnected by pivot means, shown as round pins. Load blocks, similar to those of Van Doorne et al--'113, encircle the links; however, the load blocks are constrained against longitudinal movement along the chain by the pivot means.
U.S. Pat. No. 4,493,679--Miranti, Jr. teaches a belt structure wherein a tension member 43 is positioned in a window 36 of a load block. However, the load block provides a curved surface to centrally contact the tension member.
The push belt described above is relatively expensive to manufacture and must be installed and/or replaced as a complete, endless loop. Therefore, belt repair, maintenance or initial assembly require disassembly of parts of the pulley transmission.
The pull belt offers a less expensive alternative to the push belt as no precise matching of carrier parts is required. The pull belt can be assembled with a finite length, positioned around the pulleys, and thereafter the ends are connected by a pivot member to form an endless belt. Thus, disassembly of the pulleys or transmission components is not required either for initial installation or for replacement of a belt.
Theoretically a load block entering a pulley, either on a push belt or a pull belt, is radially oriented with respect to the pulley. However, during operation the block may tilt and enter the pulley at some other angle. When the block's "window" or "windows", i.e., the opening or openings in which the block carrier is located, are made with essentially square defining edges, the tilting of the blocks causes the top and/or the bottom edges (toward the pulley and away from the pulley, respectively) to dig into and damage the carrier, seriously affecting the carrier's torque bearing and transmitting ability. As a consequence, the damaged carrier may prematurely fail. An unproven, proposed solution to the block tilt problem is to slightly round the top and bottom window defining surfaces from front to back which increases the belt manufacturing costs.